The present invention relates to new diamino compounds and liquid crystal aligning films. More particularly, the present invention relates to new diamino compounds, and to dinitro compounds and diol compounds which are intermediates of the diamino compounds, and to liquid crystal aligning films having a high pretilt angle that is obtained by using the diamino compounds.
The main current of liquid crystal display devices which are used in conventional clocks, watches and electronic calculators is a twist nematic (abbreviated as TN hereinafter) mode having a structure in which molecular alignment of nematic liquid crystals is twisted at an angle of 90 degrees on the surface of a couple of upper and lower electrode substrates. However, when the TN mode is driven by high duty, it is insufficient to obtain improved display in quality and size because it shows an indistinct contrast and a narrow viewing angle.
Since a liquid crystal display device using super twisted birefringence effect (T. J. Scheffer and J. Nethriug, Appl. Phys. Lett., 45 (10), 1021 (1984)) has been reported, a liquid crystal display device using supertwist nematic (abbreviated as STN hereinafter) mode in which the molecular alignment of nematic liquid crystals is twisted at angles of 180-300 degrees between the upper and lower electrode substrates has been developed and then large panel liquid crystal display devices having excellent display quality are developing.
It is insufficient to only align the liquid crystal molecules of aligning films used in such liquid crystal display devices. To secure good responsibility and bistability, it is necessary to have a certain angle, the so-called pretilt angle between substrate surfaces and liquid crystal molecules. It is desired to widen the pretilt angle in proportion to the twist angle. In aligning films having a relatively narrow twist angle (twisted at 180-200 degrees), it is sufficient to have pretilt angles of three degrees and below. In aligning films having a twist angle of 200-300 degrees along with better display quality, it is necessary to use the aligning films having uniform and higher pretilt angles (5 degrees&lt;.theta..ltoreq.10 degrees ) covering the broad display surface of a substrate.
There are also polyimide aligning films having high pretilt angles for the STN mode. Problems of these films are uniformity and reproducibility of pretilt angles over the whole display surface of a substrate.
In order to obtain the high pretilt angles definitely, the best mode which is currently conducted is thin film formation by vacuum oblique evaporation of SiO and the like. However, as the films are mass-produced by the vacuum oblique evaporation, it is a costly process in its production unit.
When the same scene of a liquid crystal display device is lighted up for a long time, an afterimage phenomenon may be found after the scene has disappeared. It is considered that the phenomenon is due to electric double layers which are produced on the aligning film surface by ion components of impurities contained in the liquid crystal since DC components are applied on the liquid crystal display device, electric charge differences which are produced between upper and lower substrates, and electric potential differences from the stable electric charge differences. Particularly, in case of TFT devices, since DC components can not be removed for characteristics of the devices, the after-image phenomenon is more conspicuous and serious than that of TN and STN.
Accordingly, by using a simple method employed in aligning films of usual TN modes, namely by surface treatment of rubbing organic thin film, it is desired to realize the aligning film which does not show the after-image phenomenon, and can realize good aligning characteristics and high pretilt angles having uniformity and reproducibility.
Japanese Publication of Unexamined Patent Application No. 61-240223 discloses a liquid crystal display device that is equipped with a liquid crystal aligning films produced from a polyimide resin having a repeating unit represented by a formula: ##STR2## wherein R1 and R2 indicate an alkyl group having 1 to 3 carbon atoms, and R3 and R4 indicate hydrogen or an alkyl group having 1 or 2 carbon atoms, respectively.
As raw materials for the polyimide resin, an embodied example is disclosed by using a diamine represented by a formula: ##STR3##
However, the polyimide aligning films produced from the above diamine have a problem that high pretilt angles are unobtainable as shown in the comparative examples described hereinafter and the afterimage phenomenon is found.